Graph API Subscription with Azure AD Graph API

Question

Whenever I want to post a subscription, I get the following Not-Found-Error:
{"odata.error":{"Code":"Request_ResourceNotFound","Message":{"Lang":"en","Value":"Resource not found for the segment 'subscriptions'."}

This is my POST-Request:
POST https://graph.windows.net/{tenant-id}/subscriptions?api-version=1.6

{
   "changeType": "created",
   "notificationUrl": "https://my.service/api/notifications/endpoint",
   "resource": "users",
   "expirationDateTime":"2021-04-26T18:00:00Z",
   "clientState": "secretClientValue"
}

Are there any limitations for the Azure AD Graph API?